In a wireless relay system using an orthogonal frequency division multiplexing (OFDM) method, there is proposed a technique in which, assuming that relay is performed in up to three stages in one frequency channel, one frequency channel is divided into three segments, and different segments are used in respective relay sections (See, e.g., Patent Document 1). In the technique disclosed in Patent Document 1, one frequency channel (channel bandwidth) is divided into a plurality of segments, and relay is performed in communication between base stations through a plurality of relay stations. This technique results in effective use of frequency.
FIG. 6 shows an example of a configuration of a wireless communication system 201 using the three-stage relay scheme. FIG. 7 shows an example of a frame format used for the three-stage relay scheme. In this example, one frequency channel is divided into three segments seg0, seg1 and seg2 in a frequency-axis direction. In one frequency channel, a DL (downlink) section is first arranged in a time-axis direction, and a UL (uplink) section is arranged with a predetermined guard gap between the DL section and the UL section.
In the DL section, after a preamble region of the entire frequency channel, a frame control header (FCH) and a DL/UL MAP region are provided in the segment seg0. Subsequently, a burst DL data region is provided. Further, in the UL section, a burst UL data region is provided subsequent to a control region. In the DL/UL MAP region, allocation information of the DL direction and the UL direction is included. The segments seg1 and seg2 have the same configuration as that of the segment seg0.
In the wireless communication system 201, a base station 210 includes one mobile station (MS) 211, which is a radio unit having a function of mobile station. A first relay station 220 includes two base stations 221 and 222, each of which is a radio unit having a function of base station). A second relay station 230 includes two MSs 231 and 232, and a terminal station 240 includes one BS 241. The MS is the radio unit having a function of the mobile station and performs wireless communications (transmission/reception) with the BS. The BS is a radio unit having a function of the base station and performs wireless communications (transmission/reception) with the MS.
The segment seg0 is used in the first relay section 261 as formatted in a frame format 251. The segment seg1 is used in the second relay section 262 as formatted in a frame format 252. Further, the segment seg2 is used in the third relay section 263 as formatted in a frame format 253. Thus, it is possible to perform the relay on the same channel.
Here, a system in which a throughput either in an upstream direction or in a downstream direction is emphasized so that the upstream throughput and the downstream throughput are asymmetric is required depending on a higher-hierarchy application. However, in a system in which the relay is not performed, a time period ratio of the DL section to the UL section (Hereinafter also referred to as “DL:UL”) is simply changed in response.
Patent Document 1: Japanese Patent Application Publication No. 2013-098963
However, in the above relay system, the DL section and the UL section are provided in reverse directions to the upstream direction and the downstream direction in each relay section. Further, in order to avoid an interference, the same ratio of the DL section to the UL section (DL:UL) needs to be used in each relay section. Therefore, there may be a problem that it is difficult to have a configuration in which emphasis is placed on the upstream throughput or the downstream throughput.
Further, since the relay system is configured to perform three-stage relay at maximum, two segments are used and one segment is wasted when the relay system performs two-stage relay, which is problematic.
In the technique disclosed in Patent Document 1, it is possible to reduce degradation of reception quality due to loop interference of transmission wave. However, the technique disclosed in Patent Document 1 does not consider the aforementioned problem, so that another technique is required to solve the aforementioned problem.
The present invention has been made in view of such conventional problem, and it is an object of the present invention to solve the above problem.